Those modifiers that are reflexive, do they count when the attack is unopposed?

I lean towards yes, but depending on what the modifiers are actually for. For example, I don't think the re-arming or standing back up modifiers would apply if that character makes no attempt to fight, as they seem to derive from the "multiple actions" concept.

What do you think? Which common reflexive modifiers do you think apply (to the attacker, I.e the + part) when the attack is unopposed?

I give the bonuses for the attacker even if the defender is not defending against the attack. To do it otherwise is a bigger immersion break for me. How on Earth can it be HARDER to hit someone who is not even trying to defend himself? And don't tell me that he is dodging, since that is another action. Also, it means that there are cases when it would be much better for the defender to simply not defend, in order to ensure that the attacker doesn't get an increased chance of a critical. Again, this would be a bigger problem for me.

So in the case where knight A is fighting against B and C (all have skill 15), and needing to rearm, he can ignore C, focusing more on B. But this does mean that C still gets +5 to his skill to attack A, and likely scores a hit. Which is how it should be, IMHO. This is not exactly your situation, since A is still doing a combined action by fighting B.

(This has roots in the fact that Pendragon system uses a single roll for both attack and defense, so they get bundled up. Overwhelming majority of the time, it is not a problem. But occasionally you end up with a situation where the rules need some bending.)

In the above case (fighting two opponents) I would agree like you say, a combined action is taking place. Interesting discussion! Some more thoughts below :-)

Hm. I'm not so sure the unopposed option is necessarily better, it really depends on circumstances, which makes it more of a tactical choice than a no brainer. I tend to like tactical choices as long as they aren't clear cut.

In the situation with a standard Saxon warrior versus a standard PC knight (19-20 weaponskill) the numbers would look somewhat like this.

1. Unopposed, Saxon 70% chance to do damage, 5% risk of a critical. Con; no chance to receive shield bonus to armour, or winning the combat for that matter.

Or

2. Opposed, Saxon 95% chance of succeeding, 5% risk of crit. However, 70% chance of shield bonus. Calculating the chance of rolling equal or higher on a skill of 14 ( modified knight) vs a skill of 19 (modified saxon) gets really tricky. Given that 5 results on the part of the Saxon results in only 5% chance (a crit) and the rest are a about even, I'd put it around 25%.

An interesting situation, I think. :-)